Cryopumps

ABSTRACT

This invention relates to cryopumps, and consists in the incorporation of walls for trapping by condensation exemplified by outer faces of partitions, and walls for trapping by adsorption, exemplified by inner faces of these partitions. These walls define a confined area in the closed position, in which the adsorbent is isolated from condensable gases. Once these gases have been condensed on the said outer faces, the partitions are opened to place the adsorbent layers in communication with the remainder of the pumping enclosure. 
     The invention is applicable to the production of high and uncontaminated vacua (in particular for the production of thin films).

BACKGROUND OF THE INVENTION

The present invention relates to cryopumps of the kind which incorporate, within a thermally insulated enclosure, trapping means comprising refrigerated walls for trapping by condensation, and walls for trapping by adsorption which are covered with at least one layer of adsorbent substances such as activated carbon or zeolites. By combining these two trapping means it is possible, at least in theory, to achieve extremely high vacua, of the order of 10^(-x) Torr, by virtue of the fact that gases which are difficult to condense even at very low trapping pressures, such as hydrogen and neon, are trapped by the walls for trapping by adsorption. However, the effectiveness of the walls for trapping by adsorption is considerably reduced by the fact that they become contaminated by more easily condensed gages such as nitrogen, oxgen and argon. It has proved possible to alleviate this drawback by placing the walls for trapping by adsorption downstream of the walls for trapping by condensation with reference to the flow of the gases, which latter walls also advantageously include a wall forming a baffle which shields the walls for trapping by adsorption from direct impingement of the gases.

A baffle of this kind has at least three disadvantages. Firstly, it is expensive to produce, secondly, if it is to be efficient under molecular conditions it needs to be optically sealed, which results in a considerable reduction in the pumping rate of the adsorbent covered surface, and finally, no matter what its design, it performs virtually no protective function for the adsorbent then the conditions under which the gases flow in the enclosure are viscous or intermediate, which may occur in certain applications of the cryopump.

Another method of pumping by adsorption has also been proposed which consists in depositing on the cryosurface a gas which condenses at the temperature concerned and which as the property of being adsorbent to the incondensable gases (a deposit of CO₂ for example has the property of being adsorbent to hydrogen at 20° K.). Firstly, this deposit of adsorbent gas suffers from the same drawbacks as a solid adsorbent, as described above, and secondly, when the adsorbent gas is injected onto the cryosurface there is always a rise in pressure in the enclosure either as a result of an excess of injected gas or as a result of surface heating of the cryodeposit already present, and this rise in pressure is generally undesirable.

A first object of the invention is to provide a sorption cryopump having a high pumping efficiency, i.e. a maximum pumping rate under molecular conditions and good protection for the adsorbent under viscous or intermediate conditions.

A second object of the invention is to provide a cryopump in which adsorbent gas is injected but any rise in pressure in the enclosure when the adsorbent gas is injected is prevented.

SUMMARY OF THE INVENTION

In order to achieve the above and other objects, the invention provides a cryopump which includes partitioning means which define a confined area and which comprises at least one partition which is moveable under the prompting of a control means accessible from outside the said enclosure, from a closed position in which the confined area is isolated from the remainder of the enclosure, to a position in which there is communication between said confined area said remainder of the enclosure, the layers of adsorbent substance being situated in said confined area when said partitioning means is in said closed position.

In this way it is possible to preserve the walls for trapping by adsorption in the fully regenerated state in all the phases preliminary to that in which they finally come into action to capture the last residual amounts of gases which are very difficult to condense. Since they are not exposed to the more easily condensed gases, which are trapped by the condensing walls, the effectiveness of the walls for trapping by adsorption remains complete and undiminished.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention wall become apparent from the following description, which is given by way of example with reference to the accompanying drawings in which:

FIG. 1 is a partial diagrammatic perspective view, of which part is exploded, of a cryopump according to the invention in the initial pumping phase,

FIG. 2 is a view of the cryopump shown in FIG. 1 in the final pumping phase,

FIGS. 3 and 4 are views similar to FIGS. 1 and 2 of a modified embodiment, and

FIGS. 5 and 6 are more detailed views, in elevational cross-section and from above, respectively, of the control arrangement fitted to another, modified embodiment of cryopump.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, and firstly to FIGS. 1 and 2, in a pumping enclosure 1 of which a part of the wall 2 is shown, there is positioned a cryopump body formed by partitioning means 3 made up of a plurality of longitudinal partitions 4 (eight in number) which extend in an arrangement of polygonal outline between two end partitions 5 and 6. The partitions 4 are mounted on longitudinal axes defined by lugs 7 and 8 which engage in pivot bearings in the end partitions 5 and 6. Underneath the end partition 5, a ring 9 is mounted to rotate circumferentially, which ring may be caused to rotate by a linkage 10 which passes through the enclosure wall 2 via a sealed joint 11. The ring 9 is secured to a circumferentially pivoting part 12 and part 12 is mechanically connected to each partition 4 by a hinged link 13 so that, starting from the fully closed position shown in FIG. 1, traction on the linkage 10 in the direction of arrow f1, the effect of which is to produce a circumferential rotating movement in the direction of f2, produces, via links 13, a rotary opening movement which produces extensive communication between a confined area 20 which is defined by the partitions, 4, 5 and 6 when in the closed position, and the remainder of the enclosure 1.

The partitions 4 have the particular feature of being metallic and are preferably of a metal which is thermally conductive. They are coated on only one face with a layer 14 of adsorbent substance which is generally in the form of particles of activated carbon or zeolites which are bonded to the metal surface or embedded in the metal. Refrigerating means, such as welded tubes, carry helium at 20° K. to enable the partitions 4 to be cooled. In the present embodiment, the inside faces (FIG. 1) of the partitions and their coating 14 of adsorbent substance form the wall for trapping by adsorption, while the outside faces of the same partitions 4, which have no coating, form the wall 16 for trappiang by condensation.

The cryopump described above operates in the following manner:

A chamber to be evacuated (not shown) and the cryopump are first bought down to a primary vacuum (a pressure of less than 5.10⁻¹ Torr) by means of a mechanical pump, the cryosurface formed by the partitioning means 3 being in the open position (FIG. 2) during this operation.

The cryopump is then cooled, the partitioning means 3 being moved to the closed position (FIG. 1). The chamber is then evacuated by the cryopump, the condensable gases being trapped by the outer faces 16 of the partitions 4, which are free of adsorbent. The limiting vacuum so obtained is identical to the vacuum obtaind by a cryopump and depends on the amount of incondensable gases in the the chamber to be evacuated. Then, either to reduce the residual level of incondensable gases or because the experiment going on in the chamber releases incondensable gases, it is possible to act on the linkage 10 to cause the partitions 4 to pivot. The adsorbent which is affixed to what were originally the inner faces 14 of the partitions 4 is then allowed to communicate with the enclosure 1 and becomes the chief factor in the pumping action. The angle to which the partitions 4 are opened may be adjusted, thus allowing the pumping rate for the gases to be adsorbed and the level of contamination by the other gases to be varied simultaneously.

In the modified embodiment which is shown in FIGS. 3 and 4, the geometrical arrangement of the partitioning means 3 is the same but in this case the inner faces of the partitions 4 do not have a permanently affixed adsorbent substance and opposite each of the partitions 4, when they are in the closed position (FIG. 3), is one of a number of slots 30 which are formed in the periphery of a hollow shaft 31 whose interior communicates with a duct 32 connected to a source of gas under pressure (not shown) which is situated outside the cryopump.

The operation of this modified embodiment is the same as was described above except that in the final phase when the walls for trapping by adsorption come into action, but before the partitions 4 are opened, amounts of adsorbent gas are fed through the slots 31 amd immediately condense at 33 on the inside faces of the walls 4 for trapping by adsorption. On the deposit of gas has been applied, the linkage 10 can then be operated to cause the walls 4 to rotate as above.

In the modified embodiment which is shown in FIGS. 5 and 6, there are again present partitions 50 (eight in number) which are arranged in a polygonal outline and of which an inner face 51 is covered with adsorbent substance while an outer face 52 is free of any covering. As before, the partitions 52 are mounted to hinge on shafts 53 and are capable of being turned through an angle of 90° by a linkage 10 which passes through a sealed joint 11 in the enclosure 2 and via a chain 56 which engages with sprockets 57 secured to the shafts 53. In the present case the surfaces for trapping by adsorption have been considerably increased by providing an equal number of fixed radial partitions 54 which extend from a central shaft 55 to the immediate vicinity of the partitions 50. These radial partitions 54 are covered with adsorbent substance on both faces.

The applications of the present invention are ones where a high, clean vacuum is required at high pumping speeds, and in particular applications where large amounts of incondensable gases are released (such as hydrogen in the case of thin-film production, controlled nuclear fusion, etc).

The best known example of such an application is to apparatus for thin-film evaporation, where returns to ambient pressure are frequent and there are thus many periods of operation under viscous conditions (5.10⁻¹ -10⁻⁴ Torr). The invention enables virtually all of the disadvantages attendant on the returns to ambient pressure to be overcome. In this case the saturation of the adsorbent (in the case of a solid adsorbent) is due only to the amounts of incondensable gases released during evaporation. 

We claim:
 1. A cryopump comprising a thermally insulated enclosure, means defining a confined area within said enclosure, means defining cold walls for trapping by adsorption in said enclosure but outside said confined area, means defining adsorbent layers within said confind area, and means to open and close said confined area whereby when said confined area is closed, said cold walls trap by condensation, and when said closed area is thereafter opened, said adsorbent layers trap by adsorption.
 2. A cryopump as claimed in claim 1, said confined area being defined by partitioning means whose outer walls comprise said cold walls and whose inner walls support said layers of adsorbent substance.
 3. A cryopump as claimed in claim 2, in which the inner sides of said partitioning means are permanently covered with said layers of adsorbent substance.
 4. A cryopump as claimed in claim 2, and means to introduce an adsorbent gas within said confined area to condense on the inner sides of said partitioning means when said confined area is closed.
 5. A cryopump as claimed in claim 1, said confined area being defined by partitioning means having polygonal cross-sectional configuration when closed, at least one of the sides of said partitioning means being rotatable about a central longitudinal axis.
 6. A cryopump as claimed in claim 1, said means for opening and closing said confined area comprising a linkage which slides longitudinally in a sealed passage through a wall of said enclosure.
 7. A cryopump as claimed in claim 1, said means supporting said layers of adsorbent substance comprising a radial array of walls within said confined area. 